For the above type of QAM receivers it has been relatively easy to develop a demodulator for a signal whose symbol rate is known with a reasonable accuracy. Components operating this way have in fact been commercially available for some time. Generally speaking, however, the transmission of a digital television image is just being introduced and, thus, the pertinent standards and interpretations thereof are still somewhat open. In the prior art solutions, the symbol rate has been usually identified by means of a feedback loop which, if necessary, has employed the feedback of possibly demodulated symbols.
Hence, in digital television image transmission, especially in a cable network, the modulation method used currently is a multi-state QAM, having a spectrum which is quite sharp-edged and, thus, the signal itself contains quite a bit of synchronization information. A result of this is that the carrier frequency and symbol rate of a signal must be set at a relatively high accuracy, before the synchronization based on a feedback loop is able to lock itself. In digital video, the bandwidth of a signal is typically about 7 MHz and the locking range is a few tens of kHz's. In case the symbol rate is unknown or may vary over a wide range, the correct rate must be searched in current solutions by adjusting the locking at various rates, which are generally quite close to each other at equal intervals. Therefore, at a fluctuating symbol rate, the searching time will often be unacceptably long and tedious.